major problems importing timelines

I've used Encore numerous times to author dvd's but on a current project where I'm behind schedule I encountered major problems. When I tried to import some timelines received following error messages --

"OS Status error =-37" and another stating an obvious file "...was not found." What's that all about? The files are located on an external firewire drive and play properly when I open them outside of Encore. Any suggestions?